The present invention relates to transponders, and, more particularly, to the structure of transponders or "tags" which comprise, as a minimum, a substrate, an antenna arranged on the substrate and a signal processing chip (preferably a surface acoustic wave or "SAW" device) coupled to the antenna.
Transponder tags of the aforementioned type receive an interrogating signal from a remote transmitter; process that interrogating signal to generate a reply signal; and transmit the reply signal to a receiver and decoder which receives the transponder generated reply signal and decodes that reply signal for further use. Each transponder is provided with unique encoded information that is incorporated into the reply signal. Such transponders are generally referred to as "passive" because they do not carry a self-contained power source, such as a battery, that must be replaced from time to time. Transponders of this type are described in U.S. Pat. No. 4,725,841 for "System For Interrogating A Passive Transponder Carrying Phase-Encoded Information"; U.S. Pat. No. 5,095,240 for "Inductively Coupled SAW Device And Method For Making The Same"; U.S. Pat. No. 4,737,789, for "Inductive Antenna Coupling for a Surface Acoustic Wave Transponder", and U.S. Pat. No. 4,951,057 for "Inductive Input/Output Coupling for a Surface Acoustic Wave Device". These transponders are used, for example, as personnel identification tags and for vehicle identification on highways and parking lots.